Article pubs.acs.org/Macromolecules
Unexpected Solvent Influence on the Rheology of Supramolecular Polymers Bruno G. Alvarenga,† Matthieu Raynal,‡ Laurent Bouteiller,*,‡ and Edvaldo Sabadini*,† †
Department of Physical-Chemistry, Institute of Chemistry, University of Campinas − UNICAMP, Campinas, Brazil Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut Parisien de Chimie Moléculaire, Equipe Chimie des Polymères, 4 Place Jussieu, F-75005 Paris, France
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S Supporting Information *
ABSTRACT: A very limited change in the nature of the solvent (toluene versus pentylbenzene) can have an unexpectedly strong effect on the viscosity of a hydrogenbonded supramolecular polymer. In pentylbenzene, slightly stronger hydrogen bonds are formed than in toluene, but the supramolecular polymer solution is ca. 25 times less viscous. Several hypotheses can be envisaged to explain this counterintuitive result. We propose that this effect is actually related to a solvation effect involving the outer corona of the supramolecular objects. When the cohesive energy density of the solvent approaches the cohesive energy density of the outer corona of the supramolecular objects, the viscosity of solution is reduced, possibly because of faster local dynamics.
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cooperative growth15 or chiral amplification effects16 that can conveniently be probed by spectroscopic tools and that sometimes even result in macroscopic property changes. Moreover, the absence of chain bundling avoids the complications associated with kinetic traps that can occur during the growth of other large size systems such as crystalline fibers. The observation that a particular hydrogen-bonded supramolecular polymer (see below) forms more viscous solutions in toluene than in dodecane (see Figure 1) although dodecane is significantly less polar triggered our interest. We report here a
INTRODUCTION The choice of the solvent has a strong influence on the outcome of any self-assembly process, which makes the rationalization of solvent effects of paramount importance. In the case of out-of-equilibrium processes such as crystal engineering,1−3 organogelation,4−6 or the processing of nanostructures,7 such rationalization is particularly challenging. At first glance, the case of systems that are at thermodynamic equilibrium seems to be much better understood. For instance, in the case of hydrogen-bonded assemblies, the main solvent characteristic is known to be its polarity. The influence of the hydrogen bond competition by the solvent polar groups can even be quantitatively accounted for.8,9 Still, when considering solvents of similar polarity, subtle effects can occur that significantly alter the outcome of the assembly structure and properties. For example, the bulkiness of aromatic solvent molecules has been shown to control the stability of tubular supramolecular assemblies because of the necessity for the solvent to fit inside the cavities.10 Similarly, replacing a cyclic alkane solvent by a linear alkane has been shown to change the inner conformation or even the helical sense of self-assembled rods,11−13 possibly because the thinner linear alkanes are able to intercalate between the alkyl chains of the stacked monomers. Even the length of alkane molecules has been shown to influence the stability of stacked hydrogen-bonded dimers through a strong odd−even effect.14 A better understanding of these and possibly other subtle solvent effects is clearly required before more complex issues such as the influence of solvent on crystal growth can be envisaged. Supramolecular polymers, i.e., long chains of noncovalently and reversibly linked repeat units, are ideal tools to study such effects because their large size can reveal © XXXX American Chemical Society
Figure 1. Structure of EHUT (a). Photographs of EHUT solutions in toluene or n-dodecane (b) 10 s (left) and 2 min (right) after inverting the vials (20 mmol L−1, 20 °C). Received: April 15, 2017 Revised: August 18, 2017
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DOI: 10.1021/acs.macromol.7b00786 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
scan. The transition temperature (T**) was taken as the average of heating and cooling scans, at a scan rate of 1 °C min−1. Isothermal Titration Calorimetry. The EHUT heats of dissociation were measured using a MicroCal VP-ITC titration microcalorimeter. Aliquots (5 μL) of 0.25 mmol L−1 EHUT solutions in a syringe were automatically injected into a continuously stirred (310 rpm) isothermal cell (1.420 cm3) containing the pure solvent.
careful study of this effect and show that within the family of alkylbenzene solvents an increase in the alkyl chain length leads not only to stronger hydrogen bonds but also to much less viscous solutions. We discuss possible interpretations for this highly counterintuitive effect.
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EXPERIMENTAL SECTION
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Bis-urea EHUT was synthesized as previously described.17 All aliphatic and aromatic solvents were obtained from Sigma-Aldrich and used without purification. The amount of water (
